An electric vehicle includes an electric traction battery which provides electric power for an electric traction motor, or an induction motor, which in turn drives the wheels of the electric vehicle. The traction battery is often made up of a plurality of modules connected in series. The modules are typically one or more interconnected battery cells.
Induction motor controls are used for electric vehicles including an AC induction motor, an electronic power inverter, and a micro-processor based controller. The inverter generally includes six switching transistors whose on/off stage will convert a DC current provided by a power supply, such as a DC battery, into an AC current required by the induction motor. The maximum voltage available to the motor is limited by the battery voltage and the maximum current available to the motor is limited by the current carrying capability of the switching devices.
Typically, an induction motor drive provides three stages of operation. At low speeds, the voltage required by the motor is lower than the voltage capability of the inverter. The output torque is limited by the current capability of the inverter, which is independent of the speed. Accordingly, the first stage of operation below a base speed is often called the constant torque stage of operation. At a medium speed range, above the base speed, the maximum torque can only be achieved when the motor is operated at both voltage and current limits. In this stage, the maximum output torque is inversely proportional to the speed, hence it is called constant power stage. At high speeds, the voltage capability of the inverter is the primary limiting factor for the output torque. The maximum torque is inversely proportional to the square of the speed. It is referred to as the voltage limit stage.
In a typical electric vehicle, the traction battery voltage will be dependent upon the state of charge of the traction battery. Typically, a traction battery at higher states of charge, i.e. a freshly charged traction battery, will exhibit a higher traction battery voltage. Likewise, a traction battery at lower states of charge, i.e. a depleted traction battery, will exhibit a lower traction battery voltage.
In an electric vehicle drivetrain, the maximum amount of torque and/or power that the electric motor can produce and deliver to the drivetrain varies based on the state of the charge of the traction battery, and more specifically on the traction battery voltage. A higher traction battery voltage will produce a larger maximum torque and/or power for the electric motor at speeds above a base speed.
Without a control system, the electric vehicle exhibits acceleration and power capability based upon the state of charge of the traction battery, which changes as the state of charge changes. The result is that the vehicle may appear to "over-perform" with a battery at higher states of charge and "under-perform" with a battery at lower states of charge. The result is inconsistent performance of the electric vehicle.
Additionally, in a typical electric vehicle, the transaxle and other mechanical assemblies of the drivetrain connected to the motor are typically designed based on maximum torque and/or power ratings. Based on the selection and characteristics of the motor and traction battery, the maximum torque and/or power which can be generated by the motor may exceed the designed torque and power ratings of the transaxle or other mechanical assemblies. Without a method of controlling this situation, it is possible that the motor and traction battery may be selected in such a manner so as to cause damage to the transaxle or other mechanical assemblies, which may subsequently lead to a shortened life span for the mechanical assemblies.